The platelet integrin, aIIb3, interacts with adhesive ligands such as fibrinogen and fibrin, and mediates platelet adhesion and aggregation. Integrin aIIb3 thus plays a critical role in the development of thrombotic diseases such as heart attack and stroke. Integrin aIIb3 also transmits signals bidirectionally: The ligand binding function of integrin aIIb3 is activated by signals from within platelets (inside-out signaling). Ligand binding to aIIb3 induces outside-in signals, which transmit into platelets and elicit cellular response critical in amplifying and stabilizing thrombi, in wound healing and the re-canalization of blood vessels. The goal of this application is to understand the mechanisms of integrin outside-in signaling. It is known that integrin outside-in signaling requires the activation of the Src family of kinases and regulation of small GTPases such as RhoA. However, the upstream mechanisms that initiate integrin-mediated Src activation have been unknown. Furthermore, integrin aIIb3 initially mediates platelet spreading, but later induces platelet-mediated clot retraction, which requires the movement of platelet membranes in the opposite direction. It has been unclear how integrin aIIb3 switches the direction of membrane movements. During the current funding period, we have made the following discoveries: (1) The G protein subunit, Ga13, directly interacts with the cytoplasmic domain of the integrin 3 subunit, and this interaction is required for the integrin-dependent c-Src activation, RhoA inhibition and platelet spreading (Gong et al, Science, 2010). (2) a calpain cleavage of 3 disrupts the c- Src-dependent RhoA inhibition, inducing RhoA-dependent clot retraction. (3) the small G proteins, RhoA and Rac1, are both important in integrin-dependent retractile signaling and clot retraction. Based on data from these studies, we propose following hypotheses: (1) integrin aIIb3 outside-in signaling is mediated by the direct binding of Ga13 to the cytoplasmic domain of 3. (2) Ga13 transmits integrin signal to activate c-Src and induce c-Src-dependent inhibition of RhoA, resulting in inhibition of platelet retraction and promotion of platelet spreading. (3) Calpain cleavage of 3 serves as a mechanism that down regulates Ga13 signaling by inactivating the Ga13/c-Src-dependent signaling pathway that inhibits RhoA. This facilitates integrin-dependent RhoA activation and stimulates clot retraction. To test these hypotheses, we propose the following specific aims: Aim 1, to determine the role of Ga13 binding to 3 in mediating integrin outside-in signaling. In this aim, we will determine the importance of Ga13, particularly the role of Ga13-3 interaction, in various integrin signaling pathways, and determine how Ga13-3 interaction is regulated and how this interaction activates c- Src. Aim 2, to investigate the mechanism of Ga13 and integrin-dependent dynamic regulation of RhoA in controlling platelet spreading and retraction. In this aim, we will investigate the role of Ga13-3 interaction in inhibiting RhoA and stimulating p190RhoGAP, competitive inhibition of Ga13-p115RhoGEF by 3, and the mechanisms of calpain-induced late RhoA activation and retractile signaling.